The present invention relates generally to optical fibers and in particular to narrow linewidth (single frequency) high-power continuous wave (CW) or quasi-continuous wave fiber lasers and amplifiers. This invention is related to co-pending patent application Ser. No. 12/198,308 filed Aug. 25, 2008.
Stimulated Brillouin scattering (SBS) is a limiting factor in the evolution of rare-earth doped fiber lasers and amplifiers towards higher power. This phenomenon is a result of a third-order nonlinear process that causes the scattering of laser light by a travelling hypersonic acoustic grating. The latter is initiated from thermal noise. From a quantum physics viewpoint, a photon is scattered by a phonon leading to a frequency-shifted photon. Therefore, as a by-product, Doppler-shifted scattered light known as Stokes light is generated. The interaction of the Stokes light with the laser light induces electrostriction in the medium leading to further amplification of both acoustic and Stokes waves. In an optical fiber, momentum conservation requires the Stokes light to propagate in the opposite direction as the laser light. It is well-known in the art that once a certain amount of optical power is coupled into or is generated in the fiber, significant backscattered Stokes light is produced causing the performance of the fiber to degrade.
This deleterious SBS process is characterized by a gain spectrum that determines the acoustic response of the medium to the pump frequency. Measurements in silica fibers have established a Brillouin shift of approximately 16 GHz and a linewidth of approximately 40 MHz at a signal wavelength of 1-1.1 μm. The effective SBS gain can be diminished through the use of a broad linewidth seed laser. However, several important applications, including coherent beam combination for directed energy purposes, nonlinear frequency conversion in resonant cavities or in single pass configurations, gravitational wave detection, and inter-satellite communications, require the use of high power narrow linewidth optical fiber amplifiers and lasers. Therefore, there is a need for techniques that mitigate the SBS process.
The SBS threshold can be increased by decreasing the effective length of the fiber, increasing the mode field diameter (MFD), or somehow manipulating the Brillouin gain. The increase in the SBS threshold through the decrease of length is limited by either requirements for amplifier efficiency or rare-earth elements solubility in silica. Much work has been done to increase the effective area of the fiber through the use of large mode area (LMA) fibers. While conventional LMA fiber designs have been successful in delivering single mode power outputs exceeding 100 watts, there is general agreement that new approaches are required for further enhancement of power.
A variety of experimental efforts have been attempted or proposed to reduce the SBS threshold through the manipulation of the SBS gain. In U.S. Pat. No. 5,851,259 by Clayton et al., the SBS threshold is reduced by introducing a modulation in the tension applied to the fiber during the draw process. This idea was expanded on in U.S. Pat. No. 6,542,683 by Evans et al. as a permanent, non-uniform stress is imparted to the fiber core through non-uniform thermal expansion and viscosity profiles. The latter inventor shows that a simple modulation of tension during the draw process leads to a marginal increase in the SBS threshold. The technique is limited by the fact that a change in the draw tension leads to a change in the fiber diameter. The latter inventor also did not envision a fiber which could be manufactured with polarization maintaining properties.
An attractive technique used in SBS mitigation is to induce a shift in the Brillouin frequency by introducing a thermal gradient in the fiber. The frequency shift was measured by Imai et al. to be approximately 2 MHz per degree Kelvin as reported in a 1993 paper in IEEE Photon. Technol. Lett. 16, pp. 133-1337. Along these lines, in 2007 Jeong et al. reported in IEEE J. of Selected Topics in Quantum Electron., pp. 546-551, that SBS gain broadening along the longitudinal direction of the fiber due to quantum-defect heating increased the SBS threshold sufficiently enough that signals with linewidths below 1 MHz and powers approaching 500 W were obtained. Fundamentally, quantum defect heating is a manifestation of the energy difference between a pump photon and a signal photon. One can also attempt to tailor the temperature variation in the transverse direction. However, it does not appear based on theoretical analysis that this would yield any appreciable SBS suppression.
Alternatively, transverse tailoring of the acoustic properties of fibers was suggested by Bickham et al. in U.S. Pat. No. 7,082,243 and was reported to be successful in increasing the SBS threshold by Chen et al. in Opt. Express 15, 8290-8299, 2007. But so far, attempts to fully utilize a transverse acoustic gradient in conjunction with a longitudinal thermal gradient have not been fruitful. Furthermore, it is well-known to those practicing the art that, in itself, the full benefit of a quantum defect induced thermal gradient is only realized in what is referred to as a counter-pump configuration. In such a configuration, the signal light propagates in the opposite direction to the pump light. While such a configuration is beneficial in suppressing SBS, it is not well-suited for monolithic all fiber configurations due to the exposure of pump combiners to the signal light. Consequently, robust deployment of counter-pumped fibers in rugged environments is problematic.
Another important issue for the full utilization of thermal gradients is the operating temperature of the acrylate polymer in the outer cladding region or in the coating region of the fiber. Operating temperatures are typically under 200° C. Based on theoretical analysis, it is possible then to be thermally limited. In other words, the polymer material will be damaged due to the heat generated in the core prior to the SBS threshold being reached. Therefore, it is of considerable interest to explore ideas to keep the temperature from exceeding maximum operating temperature while maintaining an effective thermal gradient for SBS suppression purposes.
In 2004, Wessels et al. reported in Optics Express 12, pp. 4443-4448 on a 72 m-long fiber amplifier pumped with two seed lasers. The two seed signals were separated by twice the SBS shift. The Stokes generated light from one laser signal coupled into the second laser light, allowing the first laser signal to grow to twice the power level of a single seed amplifier. One significant drawback of this technique is the requirement that the two seed signals have to be precisely tuned. Another significant drawback is that at such a small frequency separation, a parasitic process known as four-wave mixing (FWM) is prominent, leading to the generation of several frequency sidebands. This broadening of the optical power spectrum precludes the implementation of this method in fiber laser applications that require well-defined spectra such as coherent beam combining. Alternatively, another technique was proposed in 2009 by Dajani et al. in IEEE Journal of Selected Topics in Quantum Electronics 15, pp. 406-414 whereby a broad- and narrow-linewidth seed laser signals are employed. The two laser signals are sufficiently separated in wavelengths to suppress FWM while allowing for efficient gain competition resulting, at the output end of the fiber, in the narrow linewidth signal dominating the signal output power. While this technique can result in tripling the output power of the narrow linewidth signal, the authors address neither the utility of developing a thermal gradient in the fiber in conjunction with the aforementioned technique nor provide a recipe for achieving a large thermal gradient.
While some of the foregoing patents and applications may describe techniques that can lead to improvement in the power output of narrow linewidth amplifiers, each can have limitations. Accordingly, there remains a need in the art for new methods that address prior deficiencies.